In hard disk drives (HDD) employing perpendicular recording, a magnetically soft underlayer (SUL) is used just below the surface of a disk to conduct return magnetic flux in the recording media. As recognized herein, however, the SUL can introduce unwanted noise spikes in the read back signal, owing to domain transitions in the film that is part of the SUL. Accordingly, it is important to detect disks having defective SUL during the manufacturing and assembly process, to dispose of the disk before it is incorporated into a HDD and/or to identify and correct earlier manufacturing errors.
As further understood herein, spin stand fixtures can be used during manufacture to spin a disk, with a read head positioned over the disk as it would be during HDD operation, to detect any spikes that might be present in the read back signal. As critically recognized herein, however, spikes may be introduced not just by defects in the SUL, but also by thermal asperities (TA) caused by defects in the disk surface itself. The latter defects implicate a different error mode and, hence, different corrective actions. As a consequence, it is important to know whether a spike is caused by a SUL defect or by a TA. Having made this observation, the invention herein is provided.